The invention relates to a method and apparatus for identifying a gaseous substance and determining its purity, using acoustic techniques. The invention is more particularly concerned with identification of a gaseous substance of unknown species by analysis of frequency response of a resonator containing the substance in vapor form. The invention is more specifically concerned with detection of a significant air mass in a charge of refrigerant, as may occur in the case of a leak in an air conditioning system, and also with accounting for the presence of such air when carrying out an analysis of a test refrigerant for species and purity.
In the field of air conditioning service and repair, there is a need to identify the refrigerant charge contained in a system so that the refrigerant can be properly handled for reclamation and recycling, or for disposal. In recent years, because of environmental concerns, it has become the practice for air conditioner repair shops to capture and retain the used refrigerant in a reclamation system, rather than permit it to escape into the atmosphere. Also, because of the high cost of disposal of unresuable refrigerant, and because of the high cost of fresh refrigerant, economic needs have also driven air conditioner repair shops to reclaim the refrigerant charge in a reclaimer device provided for that purpose.
For similar environmental concerns, manufacturers of automotive air conditioning systems have begun to switch over from type R12 refrigerant (dichloro-difluoro methane) to another refrigerant, R134A (1,1,1,2-tetrafluoroethane) which is believed to be gentler to the environment than R12 if it escapes to the atmosphere. Type R134A refrigerant was engineered to have thermal characteristics very similar to R12 refrigerant so that R134A based systems could be used where R12 systems are now used, i.e., in automotive air conditioning systems. On the other hand, R134A refrigerant is chemically incompatible with R12 refrigerant, and cannot be reused if one refrigerant is contaminated with the other. Also, if either refrigerant R12 or R134A has been contaminated with another refrigerant such as R22, the refrigerant should not be reused. However, if the refrigerant contains air or lubricant, the refrigerant can be deemed acceptable, because the reclaiming device can remove these impurities from the refrigerant.
Techniques of identifying a species of a fluid by means of its dielectric properties have been described e.g. in U.S. Pat. Nos. 5,150,683; 5,091,704; and 5,119,671. For example, the relative percentages of a gasoline/alcohol fuel mixture are measured by applying an RF signal to a coil submerged in the mixture. This system would not be workable for identifying which of two refrigerant species is present, or if unacceptable contaminants are present in the refrigerant.
A technique to identify and distinguish between two different refrigerant gases, based on the dielectric properties of the gases, it described in U.S. Pat. No. 5,158,747. The device of that patent can also be configured to be responsive to acoustic properties of the refrigerant vapor, by sensing changes in velocity or phase angle of acoustic waves traveling in the refrigerant vapor. However, this type of device is not precise enough to sense whether impurities are present in unacceptable levels.
The refrigerant can be tested by introducing a sample of it, in vapor form, into a resonant chamber, which in the preferred mode is a Helmholtz resonator, at a controlled vapor pressure, e.g., 2.25 psig. A Helmholtz resonator has the beneficial property of providing resonances at frequencies of a few hundred hertz in a unit of very compact size. The Helmholtz resonator can also be constructed so as to have plural resonances, if desired. In one preferred mode, the resonant chamber is formed to produce two distinct resonances, and in the preferred construction the Helmholtz resonator has first and second necks, each of a respective length and area, connecting first and second volumes. A frequency generator produces a sweep of frequencies in a band that includes the two resonances, and this sweep is applied to a transducer in one of the first and second volumes. Another transducer, responsive to vibrations in the resonant chamber, produces an output signal that varies in response to the amplitude of the vibrations in the chamber. A digital circuit responsive to the frequency generator and second transducer output determines the center frequencies for the first and second resonances and also determines the frequency width of these resonances to yield quality or sharpness factors for the two resonances. Then these center frequencies and sharpness factors are compared with stored data concerning two or more candidate species of the refrigerant, and a determination is made as to the identity of the refrigerant species of the sample, and the extent and nature of any contaminants.
A thermal sensor in contact with the chamber is coupled to the digital circuit so that it can compensate for any temperature variations. The chamber is isolated from external environmental noise.
A regulator at the chamber inlet regulates vapor pressure at the sample gas to a predetermined level, for example, at 2.25 psig. The regulator also permits the chamber to be evacuated to twenty nine inches of mercury below ambient.
The device performance is entirely satisfactory when refrigerants are pure or cross contaminated (i.e., R12/R134A mixtures). The device can also determine the presence of a large quantity of air in the system, as can happen when there is a leak in the system.
However, a small fraction of air, i.e., between 2% and 20% air, in R12 refrigerant can be mistaken as contamination by R134A. Thus, the presence of a small quantity of air can prevent the device from providing an unambiguous result unless all air is somehow paired from the system. In principle, a properly charged air conditioning system should not contain any air mass in the refrigerant. However, in practice many automotive air conditioning systems do contain a small quantity of air throughout the system, or accumulated near the test ports. This can occur for a variety of reasons, such as compressor design problems, or improper purging of the system prior to charging.
Accordingly, it is desired to circumvent the air contamination problem by simple process steps that can be carried out with the refrigerant test equipment described just above, and which will automatically adjust for the air, if any, that is present in the refrigerant charge being tested.
It is therefore an object of this invention to provide a technique for automatically determining the quantity of air present in a refrigerant charge.
It is another object to provide a technique for unambiguously identifying the species and purity of a sample of a refrigerant vapor, and to account for the presence of air therein, employing acoustical techniques.
According to an aspect of this invention, a technique can be employed to circumvent the problem of air contamination in refrigerant gases. A Helmholtz resonator, or other equivalent resonator chamber, can be employed with a technique somewhat different from that described above. By operating the resonance chamber at two different predetermined pressures, a secondary parameter is derived which is directly related to the percentage of air present in the air conditioning system. Then, by subtracting the effect of the air present on the resonance frequency, a corrected resonant frequency is derived. This corrected resonant frequency can be analyzed to discriminate between refrigerants.
In particular, the resonant test call (or Helmholtz resonator) is purged and then filled with the vapor to be tested, up to a predetermined pressure P.sub.2 which can be on the order of about 10 psig. This can be carried out with a combination of a pressure sensor and a bleed valve, or through a bistable regulator. A transducer in contact with the vapor in the resonator, and the principal resonance frequency F.sub.2 is found. Then vapor is gradually evacuated from the resonator until a lower pressure P.sub.1 is reached. This can be about 0.25 psig. Then the resonance frequency F.sub.1, is found for this pressure P.sub.1.
The slope S of the normalized frequency curve is found, indicating the change of frequency with pressure. For this, the relationship is employed: EQU S=(F.sub.2 /F.sub.1 -1)/(P.sub.2 -P.sub.1)
The value of the slope S is compared with previously calibrated values or with a calibration curve to identify the percent of air in the refrigerant sample.
From this value, the expected frequency shift for the given quantity of air present can be determined, referring to other pre-calibrated reference values or to another calibration curve. This frequency shift can be subtracted from the frequency F.sub.1 to determine a corrected resonance frequency. The species and purity of the sample gas can be determined using the corrected resonance frequency.
In some cases a two resonance chamber can be used, and two pairs of resonant frequencies can be measured, one pair at each pressure P.sub.1 and P.sub.2. It is also possible to measure resonant frequencies for three or more pressure values, thus increasing the accuracy of the calculation of the slope factor S.
The above and many other objects, features, and advantages of this invention would present themselves to persons skilled in the art from a reading of the ensuing description of a preferred embodiment, to be read in connection with the accompanying Drawing.